1. Field of the Invention
The present invention relates to raster display systems and, more particularly, to a circuit and method for reducing east-west geometry mismatch between the top and bottom of a raster display.
2. Related Art
Raster display systems are used in a variety of applications such as televisions and computer displays. FIG. 1A shows a cross-sectional side view of a conventional raster display system 100. Raster display system 100 includes an electron gun 110, a deflection system 120, and a screen 130. Electron gun 110 generates and accelerates an electron beam 115 toward deflection system 120. Deflection system 120 deflects electron beam 115 horizontally and/or vertically at screen 130. Screen 130 includes a phosphor-coated faceplate that glows or phosphoresces when struck by electron beam 115.
Deflection system 120 includes a horizontal deflection generator 122, a horizontal deflection coil 124, a vertical deflection generator 126, and a vertical deflection coil 128. Horizontal deflection coil 124 and vertical deflection coil 128 are collectively referred to as the yoke. Although not shown, horizontal deflection coil 124 and vertical deflection coil 128 are wound at a ninety-degree angle relative to one another.
Horizontal deflection generator 122 generates a horizontal deflection current signal IH. When horizontal deflection current signal IH passes through horizontal deflection coil 124, a magnetic field is created that deflects electron beam 115 horizontally. The horizontal angle of deflection (not shown) is proportional to the direction and the magnitude of horizontal deflection current signal IH. Similarly, vertical deflection generator 126 generates a vertical deflection current signal IV. When vertical deflection current signal IV passes through vertical deflection coil 128, a magnetic field is created that deflects electron beam 115 vertically. The vertical angle of deflection θ is proportional to the direction and the magnitude of vertical deflection current signal IV.
FIG. 1B is a front view of raster display system 100. Deflection system 120 deflects electron beam 115 from a left edge LE of screen 130 to a right edge RE of screen 130 to draw a first line L1. Electron beam 115 is then briefly turned off, moved downward, and brought back to left edge LE of screen 130 by deflection system 120. Electron beam 115 is then turned on and deflection system 120 deflects electron beam 115 from left edge LE of screen 130 to right edge RE of screen 130 to draw a second line L2. This process continues very rapidly so that lines L3 through LN (where N=1, 2, 3, . . . , N) are drawn thereby creating an raster on screen 130. Note that the raster drawn on the screen 130 shown in FIG. 1B is ideal since lines LN extend to, but not beyond, left edge LE and right edge RE.
FIG. 2A shows a conventional horizontal deflection generator 122. Horizontal deflection generator 122 includes a sawtooth generator 210 and an amplifier 220. Sawtooth generator 210 generates a horizontal sawtooth signal CS1 having a constant amplitude. FIG. 2B shows a waveform for horizontal sawtooth signal CS1. Horizontal sawtooth signal CS1 is amplified by amplifier 220, which outputs horizontal deflection current signal IH.
In FIG. 2B, horizontal active time tHA corresponds to the time when electron beam 115 starts drawing a line LN at left edge LE of screen 130 to the time when electron beam 115 stops drawing a line LN at right edge RE of screen 130. The horizontal retrace time tHR corresponds to the time when electron beam 115 stops drawing a line LN at right edge RE of screen 130, moves from right edge RE of screen 130 back to left edge LE of screen 130, and starts drawing another line LN.
One problem with using horizontal sawtooth signal CS1 for horizontal deflection current signal IH is that it produces a distorted raster. FIG. 2C shows a distorted raster. Lines LN at the top portion of screen 130 and the lines LN at the bottom portion of screen 130 do not extend all the way to the left edge LE of screen 130 or all the way to the right edge RE of screen 130 which produces a distorted raster; Note that the distortions may have opposite phase, depending on the particular physical implementation. In this case, the top and bottom of the raster will be wider than the middle of the raster.
FIG. 3A shows a conventional horizontal deflection generator 122′ that can be used to address the problems shown in FIG. 2C. Horizontal deflection generator 122′ includes a horizontal geometry correction circuit 310 and a horizontal deflection signal generator 320. Horizontal deflection signal generator 320 includes a horizontal sawtooth generator 322, a modulator 324, and an amplifier 326. Horizontal geometry correction circuit 310 generates a correction signal CS2. FIG. 3B shows a waveform for correction signal CS2. Horizontal sawtooth generator 322 generates a horizontal sawtooth signal CS1 having a constant amplitude (as shown in FIG. 2B). Correction signal CS2 modulates the amplitude of horizontal sawtooth signal CS1 via modulator 324. Modulator 324 outputs a horizontal deflection signal CS3 (not shown), which is amplified by amplifier 326. Amplifier 326 then outputs horizontal deflection current signal IH.
In FIG. 3B, the vertical active time tVA corresponds to the time when electron beam 115 starts drawing the first line L1 at the top left of screen 130 to the time when electron beam 115 stops drawing the last line LN at the bottom right of screen 130. The vertical retrace time tVR corresponds to the time when electron beam 115 stops drawing the last line LN at the bottom right of screen 130 to the time when electron beam 115 starts drawing the first line L1 at the top left of screen 130.
A problem with using horizontal deflection signal CS3 for horizontal deflection current signal IH is that it produces asymmetry between the top and bottom of the raster on screen 130. FIG. 3C shows such a raster. Note that lines LN at the top portion of screen 130 extend beyond left edge LE and right edge RE. This results in an east-west (or left-right) geometry mismatch between the top and bottom of screen 130.
The reason that lines LN at the top portion of screen 130 extend beyond left edge LE and right edge RE is explained below. Horizontal deflection signal CS3 includes two components: horizontal sawtooth signal CS1 and correction signal CS2. When horizontal deflection signal CS3 is amplified by amplifier 326, amplifier 326 distorts the correction signal component CS2 of horizontal deflection signal CS3. In particular, amplifier 326 distorts a discontinuous portion 330 of correction signal CS2, creating a distorted signal portion 331. FIG. 3B shows a waveform for an amplifier output signal corresponding to correction signal component CS2. The reason amplifier 326 distorts discontinuous portion 330 is because amplifier 326 has a limited frequency bandwidth. Note that amplifier 326 eventually outputs a non-distorted signal portion 333.
Distorted signal 331 causes the amplitude of horizontal deflection current signal IH to increase at the time the lines LN are being drawn at the top portion of screen 130. As a result, the lines LN that are drawn at the top portion of screen 130 extend beyond left edge LE and right edge RE. Once amplifier 326 begins to output non-distorted signal portion 333, the amplitude of horizontal deflection current signal IH is not distorted and thus the lines LN that are drawn extend to, but not beyond, left edge LE and right edge RE of screen 130. Since the lines LN drawn at the top portion of screen 130 extend beyond left edge LE and right edge RE, while the lines LN drawn at the middle and bottom portions of screen 130 extend to, but not beyond, left edge LE and right edge RE, an east-west geometry mismatch between the top and bottom of screen 130 is created.
Accordingly, what is needed is a circuit and method for reducing east-west geometry mismatch between the top and bottom of a raster display.